Many established cell lines are available for a variety of purposes in biotechnology. Some cell lines can be cultivated as single-cell suspensions, but other cell lines do not grow well without a support. The growth of a cell line that requires support is often limited by the surface area available for the cells to grow on, since many cell lines will form only a monocellular layer on the surface. In addition, some cell lines may tend to grow in clumps or aggregates in the absence of a support, which is an undesirable result when they are needed as single-cell suspensions, but more especially when the cells are to be infected with a virus or transformed with a recombinant vector, since the virus or vector may not gain access to the cells within the clump or aggregate. Thus, there can be severe problems in scaling up the cultivation of a cell line, in particular in providing enough surface area for the cells to grow on and/or avoiding clumping of the cells.
Microcarrier technology has been used to cultivate cells in culture. For example, Forestell et al. (Biotech. Bioeng. 40: 1039-1044 (1992)) disclosed extended serial subculture of human diploid fibroblasts on microcarriers using a medium supplement that reduced the need of the cultured cells for serum. Furthermore, Ohlson et al. (Cytotechnology 14:67-80 (1994)) disclosed the bead to bead transfer of Chinese hamster ovary cells using macroporous gelatin microcarriers. Finally, Hu et al. (Biotech. Bioeng. 27: 1466-1476 (1985)) disclosed the serial propagation of mammalian cells on microcarriers using a selection pH trypsinization technique.
However, in view of the problems noted above, there is a need for improvements in methods of cultivating cell lines, in methods of producing viruses for clinical uses, and, in methods of scaling up the production of viruses for larger-scale commercialization. The instant invention meets these needs and more.